This invention relates to wiring devices such as receptacles and switches such as those in common usage in commercial establishments and households.
Wiring devices are frequently mounted in metal wall boxes and for this purpose normally have a metal mounting yoke that supports the insulating housing containing the device elements. In certain instances it is appropriate, and sometimes necessary, that the metal mounting yoke be in good electrical contact with the metal wall box which in turn is connected to a good electrical ground.
For example, duplex receptacles such as those shown and described in Howells U.S. Pat. No. 3,032,736, May 1, 1962, are the type of device for which an electrical path between the mounting yoke and the box is necessary. Such receptacles are for receiving three pronged plugs including a grounding prong which has to communicate to a good electrical ground. As shown in the Howells patent, a contact element in the housing of the device for receiving the grounding prong of a plug is in secure electrical contact with the mounting yoke. While a metal screw is normally used to secure the yoke to the wall box, the screw may not make good enough electrical contact with the yoke. The yoke is normally a stamped metal member with an aperture in which the screw fastener is loosely passed. While the head of the screw makes some contact with the yoke, there may be too high resistance for effective grounding.
As shown by art such as the Howells patent, positive grounding can be provided by a terminal screw on the yoke to which a wire can be fastened and further connected to the wall box. While quite successful, increasing interest in minimizing time required for installation of wiring devices has led to proposals for "self-grounding" in which the attachment of the yoke to the box by a threaded fastener insures positive grounding. Such devices employ an additional element generally in the nature of some sort of spring for making good contact between the yoke and the screw that holds the yoke to the wall box. Examples of other patents to this general type of device are Muska et al U.S. Pat. No. 3,432,793, Mar. 11, 1969 and Winter U.S. Pat. No. 3,609,213, Sept. 28, 1971. Additionally, there is prior art to self-grounding features that do not rely on communication of the grounding path through the fastener such as Despard U.S. Pat. No. 3,185,760, May 25, 1965 and Kuether U.S. Pat. No. 3,617,611, Nov. 2, 1971.
As noted, the interest in the self-grounding feature rose from a desire to economize in installing costs. It must be further recognized that in the art to which this invention pertains simplicity and economy in the original product design are also of utmost importance. However, none of these objectives can detract from the interest of providing reliability and effectiveness in the product.
The present invention arose from an effort to provide a wiring device with a self-grounding feature obtaining the foregoing objectives of reliability, simplicity and economy.